The present invention relates to a method of manufacturing a moulding material, an apparatus for manufacturing a moulding material and a moulding material, particularly but not exclusively to a manufacturing method and an apparatus for manufacturing a (partially or fully) preimpregnated moulding material.
Historically, moulded articles or mouldings were formed from a resin material or from a resin material reinforced with a fibrous reinforcement material. Although the moulded products thus formed were satisfactory, it was difficult to guarantee the quality of the moulded products due to the difficulties in controlling the resin/reinforcement ratio. The process was consequently refined such that the supplier of the resin provided the producer of the moulded articles with a preformed, preimpregnated reinforcement material, known as a prepreg moulding material or prepreg which was ready to be applied in the mould and which had the optimal resin/reinforcement ratio for a particular application.
Fibrous reinforcement materials have been available in various forms such as continuous strands, and woven roving format. The continuous strand is a single continuous strand used in wrapping applications. Continuous strands may also be applied in a fabric format, wherein a plurality of parallel, unidirectional structural yarns or fibres are held together by a sewing stitch or knitting stitch which runs perpendicular to the axis of the structural yarns.
A composite material comprising a continuous or uni-axial reinforcement material has high compressive and tensile strength properties in one direction. Unidirectional reinforcement materials usually have their primary fibres in the 0° direction relative to longitudinal axis of the roll from which the material may be supplied. In that case the fibres are perpendicular to the length-wise direction of the fabric, or in other words, the fibres are orientated in a direction along the roll, and this is called a warp unidirectional (UD). The orientation of the fibres can also be at 90° relative to the roll length or parallel to the length direction of the fabric and then the material is called a weft UD.
Woven roving reinforcement is used for high strength laminates having a minimal thickness requirement. Woven roving comprises a plurality of continuous strands running in two directions relative to each other, and held together by weaving the one set of strands with the other. Individual strands are not uni-axial in woven roving fabric.
Many more applications of composites are possible by orientating the fibre directions in directions which are different from the 0° and 90° directions relative to the fabric's longitudinal centreline or lengthwise direction. Additionally, some applications demand high strength in more than one direction, yet not all directions. Hence, a need has developed for a reinforcing material which has variable or multiple directional strength characteristics.
Multi-axial fabrics with yarn orientations which can vary between 0° and 90° relative to the fabric's lengthwise direction are produced by machinery which has been specifically developed for this purpose. These machines comprise weaving looms and stitching heads for assembling the fabric from layers of unidirectional material with fibres in directions which are generally different from 0° and 90° directions. Other machinery for producing multi-axial fabrics is commonly known as “multi-axial weft insertion machines”.
As an example, U.S. Pat. No. 4,567,738 discloses a structural multi-axial fabric and a method for making such a fabric. The structural fabric comprises a plurality of substantially parallel uniaxial structural yarns and a secondary yarn or support yarn or web for holding the structural primary yarns in place. The structural yarns are orientated at an angle skewed from both the fabric's centreline and a line perpendicular to the fabric's centreline. A double biased fabric or bi-axial fabric is made by sewing or stitching two skewed fabrics together with the secondary support yarn. In this way, multi-axial fabrics may also be produced by stitching layers of reinforcement material together with different fibre orientations. The stitching or sewing holds the layers of structural yarn together and keeps the fibres in the desired, pre-selected orientation. This is necessary to prevent the multi-axial fabric from distorting during transport and handling.
Such a multi-axial fabric may be utilised as a reinforcement material, and the multi-axial fabric is then impregnated with resin for example in a prepreg machine after stitching to manufacture a multi-axial prepreg. The prepreg machine applies the resin to one or both sides of the multi-axial fabric reinforcement material, and the material is subsequently compressed and heated to allow the resin to impregnate the fabric.
We emphasise here that for a period of at least 20 years, in the production of multi-axial prepregs, it has been common practice to prepare multi-axial fabrics by stitching of individual layers of reinforcement material or by using multi-axial weft insertion machines, followed by impregnation of the stitched fabric in a prepreg machine.
In fact, within the composite materials industry, due to the high costs associated with the preparation of multi-axial fabrics and the complexities of producing these fabrics, fabrics manufacturers have specialised solely in the production of these fabrics.
Since the impregnation of these conventional multi-axial fabrics is also relatively complex, the fabrics manufacturers have supplied unimpregnated stitched fabrics to the resin and moulding materials manufacturers who have subsequently impregnated and supplied the materials in the form of multi-axial prepregs to the end-users.
The relatively long supply chain of multi-axial prepregs renders the end-cost of a multi-axial prepreg material relatively high. Up until now, the application of multi-axial prepregs has therefore been on a relatively small scale.
In addition, known multi-axial fabrics have various important disadvantages which have further limited their application up until now. The presence of stitching or binders affects the mechanical properties of the multi-axial reinforcement fabric, as the fibres or yarns may be damaged or displaced by the stitching process. Furthermore, drilling and machining of composite structures with incorporated stitching has been problematic due to the softening of stitchings. Usually a relatively soft polyester fibre is used for stitching which, in combination with a reinforcing material which is of a non-polyester material, can affect the quality and mechanical properties of the cured material.
Also, the impregnation speeds of fabrics produced on multi-axial weft insertion machines tend to be low. This is due to the tight bundling of yarns as a result of the stitching operation. This further increases the cost of multi-axial prepregs. Finally, the stitching and the tight bundling of the fibres make the materials inherently stiff which can affect the drape of the material. This in turn can make the lay-up of the material in complex moulds more difficult.
It is therefore desirable to provide an improved method of manufacturing multi-axial prepreg material and to provide an improved apparatus for manufacturing such a material and to provide an improved multi-axial moulding material, thereby addressing the above described problems and/or which offer improvements generally.